1. Field of the Invention
This invention relates to a video signal processing circuit, and in particular is directed to a circuit for eliminating a noise signal included in a video signal and for cancelling a cross-talk signal picked up from adjacent tracks when a a video signal is reproduced from a particular track of a recording medium.
2. Description of the Prior Art
In a typical video recording system, such as a video tape recorder (VTR), a video signal is recorded on a magnetic medium, such as magnetic tape, in successive, parallel, slanted tracks, each track generally having a field interval recorded therein and being formed of successive areas which correspond to respective line intervals of the video signal. If the video signal is a composite color television signal, recording is carried out by separating the chrominance and luminance components, frequency modulating the luminance component to a relatively higher band of frequencies, frequency converting the chrominance component to a band of frequencies which is lower than that contained in the frequency-modulated luminance signal, combining the frequency-modulated luminance signal, and frequency-converted chrominance signal and recording the combined signal in the same track. In order to avoid interference due to cross-talk during a signal reproduction operation, that is, to avoid interference due to signals which are picked up by a scanning transducer from an adjacent track when a given track is scanned, it has been the practice heretofore to provide guard bands to separate successive parallel tracks on the record medium. Such guard bands essentially are "empty" of information so as to avoid cross-talk pickup from such adjacent guard bands when a particular track is scanned.
However, the use of guard bands to separate successive tracks is a relatively inefficient usage of the record medium. That is, if the guard bands themselves could be provided with useful information, the overall recording density would be improved. Such improvement can be attained to some degree by providing two transducers for recording the combined luminance and chrominance signals, the two transducers having different azimuth angles. Hence, information is recorded in one track at one azimuth angle and information is recorded in the next adjacent track with a different azimuth angle. When the information in such tracks is reproduced by the same, respective transducers, the information recorded in the scanned track is reproduced with minimal attenuation, but because of azimuth loss, the cross-talk which is picked up from the next adjacent track is substantially attenuated. Since azimuth loss is proportional to the frequency of the recorded signals, it may be appreciated that the cross-talk due to the frequency-modulated luminance signals included in the recorded color television signals is attenuated far more than the cross-talk due to the frequency-converted chrominance signals. Also, since cross-talk attenuation due to azimuth loss is less effective as the width of the parallel tracks is reduced, it is not sufficient to rely solely on the use of transducers having different azimuth angles in order to reduce cross-talk noise when video signals are recorded in very narrow or overlapping tracks. If the cross-talk picked up from an adjacent track is not attenuated adequately, an interference or beat signal, having a frequency different from either the information signals which are recorded in the scanned track or the picked up (crosstalk signals which are recorded in an adjacent track, will appear as a beat or moire pattern in the video picture which ultimately is displayed.
Since reliance upon azimuth loss is not completely adequate for minimizing cross-talk interference cause by the frequency-converted chrominance signals which are picked up from an adjacent track, it has been proposed that such cross-talk can be reduced substantially by recording the frequency-converted chrominance signals in adjacent tracks with different carriers. For example, the phase of the frequency-converted chrominance carrier can be constant throughout successive line intervals in one track but will shift by 180.degree. from line-to-line in the next adjacent track. As another example, the phase of the frequency-converted chrominance carrier in alternate line intervals in one track will differ by 180.degree. (or .pi.) from the phase of the frequency-converted chrominance carrier in adjacent alternate line intervals in an adjacent track, while all of the remaining line intervals in adjacent tracks will have frequency-converted chrominance carriers which are in phase with each other. Because or these phase characteristics in both examples, the cross-talk interference due to the frequency-converted chrominance signals which are picked up from an adjacent track will exhibit a frequency interleaved relationship with respect to the frequency-converted chrominance signals which are reproduced from the scanned track. Suitable filtering techniques can be used to eliminate those frequency components which correspond to the cross-talk interference.
While the use of different frequency-converted chrominace carriers is an effective technique for minimizing cross-talk interference attributed to the chrominance components, there still will be cross-talk interference due to the frequency-modulated luminance components, particularly if the record tracks exhibit minimal width. One proposed solution to this problem is to change frequency of the carrier for the frequency-modulated luminance component recorded in adjacent tracks. This is carried out by using two different bias voltages superposed onto the luminance component prior to frequency modulation thereof, which bias voltages effectively determine the frequency of a frequency-modulated carrier. As one example of this proposed solution, the frequencies of the carriers differ from each other by an odd multiple of one-half the horizontal synchronizing frequency. In a signal reproduction operation, the reproduced frequency-modulated luminance signal is demodulated, and the bias voltages which had been added to the original luminance component are removed therefrom, as by subtracting locally-generated bias voltages from the recovered luminance component. Even if the cross-talk interference picked up from adjacent tracks is included in the luminance component thus obtained, the cross-talk interference can be easily eliminated by a comb filter, because the frequency of the cross-talk interference is in a frequency-interleaved relationship with that of the reproduced luminance component.
FIG. 1 shows an embodiment of a previously comb filter which is used to cancel the cross-talk signal in the reproduced luminance component. In FIG. 1, the reproduced luminance component from an input terminal 10 is first applied to a frequency demodulator 12, and then through a delay line 11 having one horizontal interval to a frequency demodulator 13. The outputs from the demodulators 12 and 13 are additively combined with each other in an adder 14, so that the reproduced and demodulated luminance components are emphasized in level, while the cross-talk signals, which are phase-inverted from line to line, tend to cancel each other out. Hence, when the reproduced luminance components are displayed on a cathode ray tube, the cross-talk signal will not be perceived by a viewer.
However, the demodulated luminance components includes the noise signal due to the magnetic recording and reproducing, in addition to the cross-talk signal. Particularly, when the width of the record track becomes narrow, the noise level will be high because the signal-to-noise ratio of the reproduced luminance component will be greatly deteriorated.
Accordingly, an object of this invention is to provide a new video signal processing circuit which can eliminate and cancel an interfering signal, such as a noise signal or a cross-talk signal included in reproduced video signal.
According to an aspect of this invention, a video signal processing circuit for eliminating an interfering signal included in a video signal comprises a delay circuit for delaying the video signal for a predetermined number of horizontal line intervals to produce a delayed video signal; a first subtracting circuit for subtractively combining the video signal with the delayed video signal to produce a first difference signal representing the difference between the video signal and the delayed video signal; a limiter for limiting the first difference signal to produce a limiting difference signal; and a second subtracting circuit for subtractively combining the video signal with the limited difference signal to produce an output signal representing the difference between the video signal and the limited difference signal whereby the second subtracting circuit produces, as its output signal, the video signal free of the interfering signal. In order to further improve the performance of the video signal processing circuit, a high pass filter can be coupled to the first subtracting circuit to pass a high frequency portion of the first difference signal to an adding circuit to combine the limited first difference signal therewith. Alternatively, a deemphasis circuit and a preemphasis circuit can be included respectively in advance of, and following, the limiter.
Various other objects, features and advantages of this invention will be apparent from the following description taken in conjunction with the accompanying drawings.